As compared to glass, an organic film has the merits of being rich in flexibility, being difficult to break and being light in weight. Recently, by making a substrate for the flat panel display with an organic film, the investigation for allowing a display to become flexible has become more active.
In general, examples of a resin used for the organic film include polyester, polyamide, polyimide, polycarbonate, polyethersulfone, acryl, epoxy and the like. In particular, since polyimide has, in addition to higher heat resistance than that of other resins, both excellent mechanical properties such as high mechanical strength, abrasion resistance, dimensional stability and chemical resistance and excellent electrical properties such as insulation properties, a flat panel display substrate prepared with a polyimide film has been developed.
Examples of the flat panel display substrate include a TFT (Thin Film Transistor, thin-film transistor) substrate. Patent Literature 1 discloses a technique for preparing a TFT array and a display pixel on a polyimide film formed on a support substrate and peeling off them from the support substrate. Moreover, Non Patent Literature 1 discloses an organic EL display in which a transparent polyimide film is used as the substrate.
Other examples of the flat panel display substrate include a color filter substrate. The color filter is necessary for allowing an organic electroluminescence display and a liquid crystal display in which white light is used as the light source to perform color display. A typical three-color color filter has finely patterned three kinds of colored pixels of a red-colored colored pixel, a green-colored colored pixel and a blue-colored colored pixel. In the case of three-color color filter, white color is obtained by additive color mixing of three kinds of colored pixels of red, green and blue. In Patent Literatures 2 to 3, there has been disclosed a color filter in which a soluble transparent polyimide film is used as the substrate, and the use of a plastic substrate having features of being light in weight, being difficult to break and being bendable has been proposed.
With regard to the flat panel display substrate and the color filter substrate described above, firstly, being high in light transmittance in the visible light region is required. Secondly, in order to prevent the deterioration in alignment precision due to heating at the time of forming the display element and light-receiving element such as a TFT and a color filter, being low in coefficient of linear thermal expansion (CTE) is required. Thirdly, the chemical resistance to a solvent used at the time of preparing the display element and light-receiving element is required. Fourthly, the heat resistance in the formation process of a TFT and a gas barrier layer is required.
As a transparent polyimide exhibiting high light transmittance in the visible light region, there has been disclosed polyimide composed of a fluorine-containing and/or alicyclic acid dianhydride such as 2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride and 1,2,4,5-cyclohexanetetracarboxylic acid dianhydride and a fluorine-containing and/or alicyclic diamine such as 2,2′-bis(trifluoromethyl)benzidine and 4,4′-diaminodicyclohexylmethane (for example, see Patent Literatures 2 to 5). However, there is a problem that in the case where these polyimide resins are used to forma transparent substrate, a large CTE value of 50 ppm/° C. or more is exhibited, and since the chemical resistance to an organic solvent is low, swelling, dissolution or cracking occurs, and the like.
As a polyimide resin exhibiting being high in transparency, being low in CTE and being excellent in chemical resistance, there has been disclosed polyimide containing trans-1,4-cyclohexyldiamine as a diamine component (for example, see Patent Literatures 6 to 10). In these Literatures, for example, a polyimide obtained from 3,3′,4,4′-biphenyl tetracarboxylic acid dianhydride and trans-1,4-cyclohexyldiamine, a copolymer of such a polyimide and a polyimide obtained from 2,2′,3,3′-biphenyl tetracarboxylic acid dianhydride, 3,3′,4,4′-oxydiphthalic dianhydride, diphenylsulfone-3,3′,4,4′-tetracarboxylic acid dianhydride, benzophenone-3,3′,4,4′-tetracarboxylic acid dianhydride and 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride and the like and trans-1,4-cyclohexyldiamine, and the like have been disclosed.